In order to reduce a driving operation of a vehicle by a driver, a vehicle drive control device for executing an automatic drive control such as a cruise control for controlling a vehicle speed to be at a constant target vehicle speed, an adaptive cruise control (ACC) for controlling the vehicle to follow a vehicle traveling in front of the subject vehicle (hereinafter referred to as a leading vehicle) and the like, is adapted to the vehicle. In the vehicle drive control device, an engine for applying a drive torque to the vehicle and a braking device for applying a braking torque to the vehicle are cooperatively controlled so that the vehicle speed reaches a target vehicle speed. More specifically, an automatic drive control electronic control unit (hereinafter referred to as an automatic drive control ECU) calculates a target driving torque so that the vehicle speed reaches the target vehicle speed. The calculated target driving torque is outputted to an engine ECU, then the engine ECU controls the engine, which serves as a driving torque generating device, on the basis of the target driving torque. Further, in the vehicle drive control device, the automatic drive control ECU calculates a target braking torque so that the vehicle speed reaches the target vehicle speed. The calculated braking torque is outputted to a brake ECU, then the brake ECU controls the braking device, which serves as a braking torque generating device, on the basis of the target braking torque.
There is a conventional automatic drive control for controlling the vehicle to be driven at a low target speed, for example, at approximately 10 km/h. For example, a vehicle drive control device disclosed in JP2004-90679A executes an automatic drive control so that the vehicle is driven at a target speed as low as about a creep speed.
Road surfaces on which the vehicle travels are divided into on-road and off-road. In a case where the vehicle travels off-road, the vehicle may become stuck depending on a condition of the road surface. In a case where the vehicle becomes stuck, if the vehicle is somewhat able to move forward or backward, the vehicle may be able to free itself from the stuck situation by performing a forward-rearward maneuver. The forward-rearward maneuver is an operation of repeatedly moving the vehicle forward and rearward at a position where the vehicle is stuck in order to increase a force acting on the vehicle. More specifically, in the forward-rearward maneuver, the force acting on the vehicle is increased by utilizing a reaction generated when the vehicle moves rearward to when the vehicle moves forward and by utilizing the reaction generated when the vehicle moves forward to when the vehicle moves rearward. In order to increase the force acting on the vehicle when the vehicle moves forward and rearward, a driver needs to perform an acceleration operation at a right timing when the reaction is generated, in addition to a shift operation towards a direction where the vehicle moves. In other words, in order to perform the forward-rearward maneuver, the driver needs to repeatedly shift a gear lever between a forward movement position and a neutral position, between the neutral position and a rearward movement position, and between the forward movement position and the rearward movement position via the neutral position.
In the conventional vehicle drive control device, when the gear lever is shifted at the neutral position, the automatic drive control is cancelled. Therefore, in the case where the vehicle having the conventional vehicle drive control device becomes stuck, the driver needs to perform the forward-rearward maneuver by manually operating the gear lever (not shown) and an acceleration pedal (not shown). In other words, in the conventional drive control device, the automatic drive control is not useful for freeing the vehicle from the stuck situation. Further, in the vehicle drive control device disclosed in JP2004-90679A, a driving torque is gradually reduced while the gear lever is set at the neutral position. Therefore, in a case where the gear lever is shifted from the neutral position to the forward movement position or from the neutral position to the rearward movement position while the driving torque by the engine still remains in a state where the gear lever is set at the neutral position, the remaining driving torque is applied to a transmission apparatus. As a result, disadvantages such that the vehicle is suddenly accelerated, a load is applied to the transmission apparatus and the like may occur.
A need thus exists for a vehicle drive control device which is not susceptible to the drawback mentioned above.